A plasma etching device having an upper electrode and a lower electrode facing opposite each other provided within an airtight process chamber has been proposed in the prior art. When implementing a process with this device, a workpiece such as a semiconductor wafer (hereafter referred to as a “wafer”) is placed on the lower electrode. Next, a process gas is introduced into the process chamber and, at the same time, vacuum drawing is implemented in the process chamber to sustain the pressure of the atmosphere inside the process chamber at a predetermined reduced level. Subsequently, high-frequency power is supplied to, for instance, the lower electrode. The application of such high-frequency power causes the process gas to dissociate, resulting in the generation of plasma. Consequently, the wafer becomes etched and contact holes in a specific shape are formed at a specific layer at the wafer, e.g., an SiO2 film layer.
The process gas used when forming contact holes at the SiO2 film layer is constituted of a gas containing, at least, a CF (fluorocarbon) gas and O2, such as a mixed gas containing C4F8, CO, Ar and O2. When C4F8 becomes dissociated, radicals such as F*(fluorine radicals) and CF * (fluorocarbon radicals) ions and electrons are generated. The SiO2 film layer is etched as a result of the competing reaction of the radicals and the ions among them. C4F8 contains carbon (C). Thus, reaction product such as carbon and CF compounds are generated during the process. The reaction product become deposited on and accumulated at the photoresist film layer formed on the SiO2 film layer, most notably at the shoulders of the etching pattern openings. As a result, the shoulders are protected from ion collisions by the reaction product. This prevents the pattern openings from becoming wider, so that small contact holes with specific dimensions are formed.
In addition, O2 is added into the process gas to minimize the occurrence of etch stops. Namely, it has been learned through experience that by adding O2 into the process gas, removal of the reaction product can be facilitated. Thus, as long as O2 is added into the process gas in a correct quantity, the extent to which the reaction product are accumulated at the bottoms of the contact holes is lessened to prevent the occurrence of etch stops. However, if O2 is added into the process gas in an excessive quantity, the reaction product having been accumulated at the photoresist film layer, as well as the reaction product having been accumulated at the bottoms of the contact holes, become removed. This will result in the shoulders becoming etched to cause an increase in the diameter of the pattern openings. Accordingly, O2 is added into the process gas in a quantity that will prevent the occurrence of etch stops but allow the shoulders at the photoresist film layer to be ground only to a relatively small degree during the etching process. For instance, if the flow rates of C4F8, CO and Ar are respectively 10 sccm (1.67×10−7 m3/s in a normal state), 50 sccm (8.33×10−7 m3/s in a normal state) and 200 sccm (33.3×10−7 m3/s in a normal state), O2 is usually added at a flow rate of 5 sccm(0.833×10−7 m3/s in a normal state).
An etching method for forming extremely small contact holes by performing an etching process on a wafer with highdensity plasma has been proposed in the prior art. However, when forming contact holes with a high aspect ratio through this etching method, charging damage may occur due to electron shading. In such a situation, it is difficult to form contact holes with a desired shape.
Now, the electron shading phenomenon and the resulting charging damage are explained in reference to FIG. 22. FIG. 22 is a schematic sectional view of a wafer W. The wafer W is constituted by laminating an SiO2 (silicon oxide) film layer 1012 and a photoresist film layer 1014 on a semiconductor substrate 1010. FIG. 22 illustrates a state manifesting the formation of contact holes 1018 at the SiO2 film layer 1012 implemented based upon a pattern 1016 formed at the photoresist film layer 1014 is in progress.
As shown in FIG. 22, electrons (e−) collide with side walls of the pattern 1016 as the contact holes 1018 are etched further and their aspect ratio becomes higher. Positive ions (I+), on the other hand, make entry almost perpendicular to the bottoms of the contact holes 1018. As a result, the side walls of the pattern 1016 and the upper inner walls of the contact holes 1018 become negatively charged. This negative charge forms an electrical field that constitutes an electron barrier. As a result, electrons having only a small velocity component along the direction perpendicular to the bottoms of contact holes 1018 become slowed down by the electrical field and are also bounced back by the electrical field, which prevents them from entering the pattern 1016. This is referred to as the electron shading phenomenon.
When the electron shading phenomenon described above occurs, positive ions enter the bottoms of the contact holes 1018 in a larger quantity compared to the quantity of electrons, as explained earlier. Consequently, the lower walls (side walls) of the contact holes 1018 become a positively charged. As a result, a problem occurs in that the direction along which the ions constituting the etching seed travel is turned toward the side surfaces of the contact holes 1018 due to the charge, to induce abnormalities in shape such as notches. This problem is referred to as charging damage.
As a solution, high-frequency power used to generate plasma is intermittently applied to the upper electrode. This lowers the electron temperature in the plasma so that the radicals are controlled to sustain a specific state. In addition, when the ion sheath becomes dissipated while the supply of the high-frequency power is cut off, negative ions and electrons are drawn to the bottoms of the contact holes 1018 by using biasing power applied to the wafer W. This eliminates the problem of the lower walls becoming positively charged and thus, charging damage is prevented. It is to be noted that the electron temperature refers to the index representing the level of the average thermal motion energy of the electrons in the plasma. In addition, the sheath refers to a space-charge layer formed around the wafer W in the plasma atmosphere.
However, even by adopting the etching method described above, the charging damage cannot be prevented if the internal diameter of the contact holes to be formed is an extremely small at, for instance, approximately 0.18 μm or smaller. In other words, the adverse effect of the electron shading phenomenon becomes more pronounced as the aspect ratio of the contact holes increases. The extent to which the lower walls of the contact holes become positively charged also increases as a result, until the positive charge can no longer be electrically neutralized by the negative ions generated through the disassociation of the process gas. In addition, electrons make isotropical entry. For this reason, while the electrons reach the lower portions of the contact holes, they still do not eliminate the positive charge problem explained above.
The present invention has been completed by addressing the problems of the prior art discussed above. A first object of the present invention is to provide a new and improved plasma processing method that enables formation of contact holes with a high aspect ratio by preventing the occurrence of etch stops without causing damage to the mask pattern.
A second object of the present invention is to provide a new and improved plasma processing method that does not induce charging damage even when contact holes with a high aspect ratio are formed with plasma and enables formation of contact holes with a desired shape.